In an opto-electronic input-scanner apparatus a source document is scanned line by line by a narrowly focused light beam. The light reflected from the scanned document is collected by a suitable photosensor to produce a corresponding video signal whose voltage is representative of the shade of darkness or lightness of the document, hereinafter called "source signal".
However, the actual offset and amplitude of the source signal is subject to variations in accordance with such factors as variations in the intensity of the illuminating light beam, drift of the photosensors due to aging or temperature change, drift in the electronic circuits, and many other factors.
Failure to correct possible offsets and amplitude variations of the source signal results in reduction of accuracy of reproduction of both background and contrast in the recorded image.
In the prior art, a calibration scan is executed, in which the unit acquires black and white samples of the source signal produced by the photosensors in response to the illumination of a black-white scanning calibration strip by the scanning beam.
Black and white are designated by the reflectance signal obtained from a "calibration strip", also called "target", and located mostly adjacent to the image scanning area. It may be remarked that in the case of a feedthrough scanner, a calibration strip may be as well throughput in the form of a calibration sheet, or it may be temporarily turned in the optical scanning path of the apparatus (as e.g. published by Hanna and Melamud, in IBM Technical Disclosure Bulletin, vol. 21. no. 4, september 1978, p. 1410).
The first half of the strip may be imprinted black, or may be absent but replaced by a control signal which turns the lighting beam off e.g. by closing the lens aperture. The second half of the strip may be imprinted white or be apertured with a clear open gate to expose a nominally white background area of the document; or a so-called "Dmin" filter, which is a filter having a density corresponding to a calibrated white reference density, may be introduced in the optical path of the scan-apparatus.
One specific solution to the above-mentioned problem regarding the signal fluctuations, is described in U.S. Pat. No. 3,952,144 and makes preliminary calibrating scans in which sequentially a known black area and a known white area are scanned, and during which a so-called "automatic background and contrast control unit" in the transmitter stores a sample of the uncorrected video input signal at a time when the known black area is being scanned, and also stores a second sample of the uncorrected video input signal at a time when the known white area is being scanned. Thereafter, during the subsequent scanning of the source document the control unit produces voltages representing the stored black and white samples and combines these voltages to correct the video input signal received during the scanning of the source document.
However, when the real density extremes are outside the range physically incorporated by the calibration strip, said correction method does not guarantee accurate and reliable reproduction of all shades of grey that are available in the source original.